The laser gain medium is the material that exhibits optical gain within a laser through stimulated emission sustained by a pump source. At high power levels, where area scaling of laser power is desirable, slabs of solid state material are typically used for the gain medium. These slabs generally exhibit strong thermal effects in the gain medium at high power levels. A strong temperature gradient may exist in the height direction of a solid-state slab gain medium, potentially causing unwanted thermal lensing and stress induced birefringence, as well as possibly causing the gain medium to crack due to thermal stresses.
Currently, composite gain media are made of doped and undoped material layers. A typical gain medium may be a doped Yttrium Aluminum Garnet (YAG) or sapphire crystal. Dopants are typically rare-earth ions such as neodymium (Nd) or ytterbium (Yb), or transition metal ions such as chromium (Cr).
Current composite laser gain media have an inner doped section sandwiched between an outer cladding of undoped material. A feature of this cladding is to thermally insulate the waste heat generated by pumping inside the doped section. This concentration of heat inside the laser gain medium produces a thermal gradient that results in a thermal lens because the refractive index of the solid-state gain medium depends on the temperature; warmer areas having steep gradients are less effective. Typically, a strong temperature gradient develops in the height direction of the medium, causing disparities in the optical path length and gain between the top, middle, and bottom portions. This heat gradient creates mechanical stresses in the gain medium that may cause optical birefringence. This combination of thermal lensing and birefringence contribute to optical phase distortion (OPD) of the gain medium in a laser resonator, resulting in degraded beam quality and reduced power extraction efficiency.
A sufficiently severe or prolonged thermal gradient, or ongoing exposure to this kind of thermal cycling, may cause or accelerate the mechanical failure of the solid state material. This may be evidenced by cracking in crystal media, permanent distortion in other media, separation of the doped and undoped layers, and other detrimental effects caused by ongoing, uneven thermal stress.
Furthermore, embodiments of doped-core slab laser media that are configured to allow direct cooling of the core, with the resonated beam axis passing through the coolant channels, cause further OPD as the beam passes through the coolant.